1. Field of the Invention
The present invention relates to an electron gun for the cathode-ray tube in which the increase in the spot size of the electron beam with the increase in beam current is suppressed thereby to produce a satisfactory resolution in the high beam current region.
2. Description of Related Art
Generally, the electron gun for the cathode-ray tube is so constructed that the electron beam emitted from the cathode is preliminarily focused by a cathode prefocusing lens unit (also called a beam-forming region or a triode section) and an early stage lens unit making up an electron lens system, and then is caused to enter the phosphor screen to focus thereon by a main lens unit constituting the same electron lens system.
This electron lens system for the conventional cathode-ray tube electron gun is generally of two types, unipotential form lens and bipotential form lens. Other composite lens systems such as a quadra potential form (hereinafter referred to as "QPF") and a multistep potential form (hereinafter referred to as "MPF") derived from combinations of the preceding two types are also known.
A cathode-ray tube electron gun having the QPF lens is shown in FIGS. 1 and 2 as an example of the prior art. FIG. 1 is a diagram showing the basic configuration of a QPF cathode-ray tube electron gun according to the prior art, and FIG. 2 is a schematic diagram showing the arrangement of the grid electrode of the electron lens system and the light paths of the electron beam in the QPF cathode-ray tube electron gun.
In FIGS. 1 and 2, numeral 21 designates a stem, and 30 a phosphor screen, with a cathode 22, first to sixth grids 31 to 36 and a shield cap 29 arranged in that order from the stem 21 side toward the phosphor screen 30. The second grid 32 is formed in an annular shape with an electron beam aperture at the central portion thereof, and the first and third to sixth grids 31, 33 to 36 in a cylindrical shape with an electron beam aperture at both ends thereof.
The second and fourth grids 32, 34 are impressed with a cut-off voltage, the third and fifth grids 33, 35 with a Focusing voltage E.sub.F, and the sixth grid 36 with a high voltage E.sub.b.
The first, second and third grids 31, 32, 33 contribute to the forming of an electron beam. The area where these grids are arranged is called an electron beam-forming region or a triode, or considering the lens function obtained therefrom, a cathode prefocusing lens unit L.sub.1.
In similar fashion, the third, fourth and fifth grids 33, 34, 35 constitute an early stage lens unit L.sub.2 making up a unipotential form lens for focusing the electron beam irradiated from the cathode 22, and the fifth and sixth grids 35, 36 constitute a main lens unit L.sub.3 providing a bipotential form lens.
The third grid 33 doubles as the cathode prefocusing lens unit L.sub.1 and the early stage lens unit L.sub.2, while the fifth grid 35 acts as the early stage lens unit L.sub.2 and the main lens unit L.sub.3, at the same time, respectively.
In this conventional QPF cathode-ray tube electron gun, as shown in FIGS. 1 and 2, the electron beam irradiated from the cathode 22 forms a crossover by being controlled by the first grid 31, the second grid 32 and the third grid 33 making up the cathode prefocusing lens unit L.sub.1, is prefocused by the early stage lens unit L.sub.2 made up of a unipotential form lens including the third grid 33, the Fourth grid 34 and the fifth grid 35, and further focused on the phosphor screen 30 by the main lens unit L.sub.3 of bipotential form leas type constituted by the fifth grid 35 and the sixth grid 36 thereby to produce a beam spot.
FIG. 3 is a schematic diagram showing three grids G.sub.1, G.sub.2, G.sub.3 making up a unipotential form lens used for a conventional cathode-ray tube electron gun of ordinary type. The focusing voltage E.sub.F is applied to the grids G.sub.l, G.sub.3 and the cut-off voltage to the grid G.sub.2.
The diameter D of the intermediate grid G.sub.2 and the axial length L thereof hold the relationship L/D&lt;2.5 to meet the conditions as a unipotential form lens. ("Journal of Applied Physics Vol. 48, No. 6, June 1977, pp. 2306-2311")
More specifically, the unipotential form lens used for the cathode-ray tube electron gun is so constructed that the L/D for the intermediate grid G.sub.2 among the three grids G.sub.1, G.sub.2, G.sub.3 constituting the unipotential lens is set smaller than 2.5. This is also the case with the conventional cathode-ray tube electron gun shown in FIGS. 1 and 2.
Some UPF cathode-ray tube electron guns are of High-UPF type in which the axial length of the fourth grid making up the early stage lens unit is increased for a reduced spherical aberration. The length L along the axial direction of the fourth grid of this electron gun is at most 2 to 2.5 times as large as the diameter D thereof.
Important factors for determining the beam spot size on the phosphor screen of the cathode-ray tube electron gun are the repulsion of electrons due to the space charge and the spherical aberration of each electron lens system. The space charge has a large effect, in the beam-forming region (triode) where the electrons have not yet been sufficiently accelerated. The main lens unit L.sub.3 where the electrons are sufficiently accelerated, on the other hand, is affected more by the spherical aberration than by the space charge.
As a conventional method of increasing the screen brightness, the beam current is increased. With the increase in beam current, however, the space charge and the spherical aberration have a synergetic effect, with the result that the beam spot size increases, thereby causing a deteriorated resolution.
As a conventional method of suppressing the effect of the space charge, on the other hand, a high-voltage source is located in the vicinity of the triode so that the electrons are accelerated sharply to reduce the emittance affecting the beam quality. The UPF cathode-ray tube electron gun and the High-UPF cathode-ray tube electron gun, for example, employ a configuration in which a high voltage is applied to the third grid 33. These cathode-ray tube electron guns, as compared with the MPF, QPF or BPF cathode-ray tube electron gun, are known to have superior beam spot characteristics in the high-beam current region. ("Development of Color Cathode-Ray Tube with Hi-UPF Electron Gun", Institute of Electrical Communication Society Technological Research Report, 1977, ED. 77-71, pp. 1 to 8)
On the other hand, a known effective method of reducing the spherical aberration of an electron lens is to increase the effective diameter of the conventional lens. In the delta-type cathode-ray tube electron gun, for example, the possibility of increasing the lens diameter permits a superior focusing performance. The in-line electron gun, which is the main stream of electron guns in recent years for its assembly ease, however, uses a self-convergence system with an electrode configuration of three electron beams aligned in horizontal direction, and therefore poses the problem of the difficulty of increasing the lens diameter. Thus, with regard to the inline electron gun, in order to reduce the spherical aberration, efforts are being made to optimize the electrode profile and the applied voltage, with emphasis placed on the development of a large-size composite lens, an expansive electric field lens, etc., as the main lens unit.
No measure has yet been taken to cope with the increase in beam spot size with the increase in beam current.